We all know what Cialis (tadalafil) does for the phallus, but what if it fought cancer? A team from Johns Hopkins University School of Medicine just started enrolling patients in a clinical trial on Cialis for treating head and neck cancer.

The irony, perhaps, is that the tumors Cialis may help treat are more and more likely to be due to oral sex—one thing Cialis certainly makes more likely. A growing number of such cancers appear to be driven by infections with the human papilloma virus (HPV), which appear to be spreading via oral-genital sex, Maura Gillison, an Ohio State cancer researcher said last week at the Damon Runyon Cancer Research Foundation Clinical Investigator Symposium in New York City.

Like other erectile dysfunction drugs—say, Viagra or Levitra— Cialis works by boosting the effects of nitric oxide, which relaxes and opens up blood flow to the penis, facilitating an erection. But the drug has a very different effect on the immune system, explains lead investigator Joseph Califano, an otolaryngologist and head and neck surgeon at Hopkins who described his new trial at the symposium. As it turns out, Cialis might protect immune cells from being led astray by cancer.

A common feature of head and neck cancers, which can occur in the mouth, throat, tonsils, voice box, tongue and palate, is their ability to suppress the body's defense mechanisms, Califano explains. One way they do this is by ramping up activity of cells known as suppressor cells that block the activity of immune cells that would otherwise attack the tumor, he says. Cialis seems to work in the opposite direction, freeing immune cells to go after the cancer.

One of the advantages of using Cialis as a cancer treatment is that it has few side effects—except for the obvious (and some would argue agreeable) effect on male reproductive equipment, says Califano. "It's very nice to have a drug that is basically very safe."

But the fact that Cialis has such an obvious secondary effect makes it hard to conduct a blinded placebo-controlled trial, Califano notes. Under ideal circumstances, study participants have no way of knowing if they are taking the drug or the placebo, but this is difficult with Cialis—at least among male patients. 

Every year some 600,000 people are diagnosed with head and neck cancer worldwide, and half of them will die from the disease within five years, according to Gillison.

The Cialis trial, which Califano hopes to wrap up in two years, will include 40 men and women with head and neck cancer. If the drug stifles cancer growth in these patients, he plans move forward with a larger trial. 

Image of a book by Damon Runyon, a journalist and short story writer who died of cancer in 1946: ©Ivan Oransky, Scientific American